1. Field of the Invention
This invention relates to a hose for an automotive air-conditioning system, and more particularly to a composite hose for conveying fluorocarbon-based refrigerent fluids, particularly dichlorodifluoromethane sold under the technical designation "Refrigerant 12", in the liquid or gaseous state. Also, this invention relates to a hose assembly using the hose of the invention.
Additionally, this invention refers to a hose for an automotive air-conditioning system, having three basic elements; first, an innermost element, known as the tube, sometimes referred to as a lining; secondly, an intermediate element, known as a reinforcement and thirdly an outermost element, known as a cover, sometimes referred to as a jacket.
2. Discussion of Background
Conventional automotive air conditioning system hoses, particularly those for use in conveying "Refrigerant 12" consist of either (1) an elastomeric compound tube, a reinforcement ply or plies and an elastomeric compound cover, or (2) a thermoplastic polymer tube, a reinforcement ply or plies and either a thermoplastic polymer or an elastomeric compound cover.
For example, U.S. Pat. No. 3,805,848 to Chrow discloses a two ply tube with an intermediate adhesive without specifically designating it as such. The inner ply (core tube) is polyamide and the outer ply (coating) is vinylidene chloride polymer, both thermoplastics. The two plies reportedly provide for the resistance to permeation of "Refrigerant 12" and the ingression of water vapor, but they result in a fairly stiff hose which is not easily intimately joined to a hose fitting to form a functioning hose assembly.
With the growing experience in automotive air conditioning system usage, some attributes of the presently available hoses have been identified by users as in need of improvement. First the ability of the wall of hose to resist the passage of pressurized "Refrigerant 12" from the air conditioning system to the atmosphere, normally called the permeability (quantified as permeation rate or effusion rate). The elastomeric compound types have evolved to the point where a marginally acceptable permeation rate exists, but only with compounds of a few select elastomers. Also it has been determined that extraordinarily heavy thicknesses must be used, particularly for the tube in immediate contact with the "Refrigerant 12", to achieve the marginally acceptable permeation rate. The resultant weight of the product is counter-productive and conflicts with the need to reduce vehicle weights to enable meeting mandated fuel economy standards. The permeation rate of the hoses utilizing a thermoplastic polymer tube, particularly when the thermoplastic material is selected from a number of suitable polyamides (commonly known as nylon) is many times less than that found for the elastomeric compound type. The thermoplastic polymer tube type is also much lighter in weight, but there are other features that make it less than ideal to satisfy the automotive industry needs. Characteristics commonly cited as reasons for not using the thermoplastic polymer tube type as a total replacement for the elastomeric compound tube type are:
1. Thermoplasticity, particularly at the elevated temperatures encountered in underhood areas of present day vehicles and in contemplated future designs, results in diminished capability to maintain acceptable hose-to-hose fitting performance at system pressures.
2. Transmission of refrigerant compressor noise into the passenger compartment is more noticeable and can become objectionable.
3. The inherent stiffness of the thermoplastic polymer hose decreases its utility in efficient routing in the now crowded engine compartments of vehicles.
Another condition encountered in automotive air conditioning systems that has recently been highlighted as a problem, is the occurrence of water vapor ingression through the hose wall from the atmosphere into the "Refrigerant 12" in the system. The presence of the condensed water vapor in the system causes corrosion in some components, reduces thermal efficiency and in some cases has reportedly initiated unwanted chemical reactions that in some way intensify the corrosion and further deteriorate the thermal efficiency.
The prior efforts to improving automotive air conditioning system hoses have not been successful at solving each of the shortcomings of conventional air-conditioning hoses, and thus have resulted in a less than ideal hose.
U.S. Pat. No. 2,899,982 by Harpfer discloses a three-ply layer in the construction of a hydraulic brake hose. This three-ply layer is an intermediate element and is described as being provided for the purpose of increasing resistance to length-wise stress. The three plies of this intermediate element are made up of longitudinally extending nylon elements (middle ply) and an elastomer coating on each side (inner and outer plies). The nylon elements are pictured to be monofilaments.